FIG. 1 is a cross-sectional view of the structure of a photodetector incorporating a superlattice structure in a light absorption layer as shown in Applied Physics Letters. Volume 47, Number 3, Aug. 1, 1985, pages 190-192. In that structure, a multiple quantum well light absorption layer 1 is buried within an intrinsic layer 2. The intrinsic layer is sandwiched between a p-type layer 3 and an n-type layer 4. Electrodes 7 are disposed on p-type layer 3 and n-type layer 4, respectively.
When the structure of FIG. 1 is reverse biased, the absorption peak with respect to wavelength for incident light in the multiple quantum well structure is shifted toward longer wavelengths by the quantum confined Stark effect. This structure provides high wavelength selectivity based upon the electric field produced by the external bias applied to the structure. However, this structure cannot distinguish between two different wavelengths in incident light when there is only a small difference between the two wavelengths.
A photodetector employing a self electro-optic effect and producing a bistable output signal as a function of input light intensity is described in Applied Physics Letters. Volume 45, Number 1, July 1, 1984, pages 13-15. However, that article does not describe any relationship between the bistable output signal and the wavelength of the incident light.
An apparatus for detecting a particular wavelength light signal in a multiple wavelength light communication system is described in Japanese Society of Electronics and Communication Engineering , Volume 63, Nov. 1980, page 1185, and illustrated in FIGS. 2 and 3. As shown in FIG. 2, light including wavelengths .lambda..sub.1 and .lambda..sub.2 is incident on a diffraction grating 8 which disperses the different wavelength components into separate beams. Those beams are respectively incident on photodetectors 9a and 9b which are positioned relative to the diffraction grating 8 to receive the respective beams and convert them into electrical signals. In FIG. 3, the light including wavelengths .lambda..sub.1 and .lambda..sub.2 is incident on a transparent plate 10 having parallel surfaces. The light of wavelength .lambda..sub.1 is transmitted through the plate and an interference filter 11 tuned to that wavelength. The transmitted light of wavelength .lambda..sub.1 is incident on photodetector 9a which generates an electrical signal in response to the incident light. The light of wavelength .lambda..sub.2 is internally reflected within the plate 10 and transmitted through an interference filter 12 tuned to wavelength .lambda..sub.2. The transmitted light is incident on photodetector 9b which generates an electrical signal in response.
Although the diffraction grating arrangement of FIG. 2 produces an acceptable spatial separation between the different wavelength components of the incident light, the geometric constraints require that the angle of the incident light be constant. The apparatus of FIG. 3 provides an inferior separation of the different wavelengths and requires additional expensive elements, namely, the interference filters 11 and 12. In addition, the arrangements of FIGS. 2 and 3 include distinct wavelength separation and detection sections which do not have wavelength tuning capability.